In a turbo-molecular vacuum pump, a rotor with attached pump blades rotates at a high angular velocity. In order to make this rotation as loss- and wear-free as possible, often magnetic bearings are employed for supporting the rotor in a non-contacting manner. The magnetic bearings can be passive, employing permanent magnets, or active, employing electromagnets of various kinds. Also combinations of passive and active bearings, as well as combinations of magnetic bearings with standard ball bearings and/or air bearings are known in the art.
The rotor essentially constitutes a rigid top having six spatial degrees of freedom (DOF). One DOF is the rotation around the rotor axis. This DOF is usually driven by an electromotor. The other five degrees of freedom should be essentially fixed. Therefore, bearings are needed to restrain motions along these degrees of freedom.
Generally, TMPs can be operated in any orientation, i.e., the rotor axis can have any arbitrary direction in space. In the following, it is assumed for simplicity that the rotor axis is vertical and that the rotor is supported in the bearing stators by an elongated shaft of cylindrical symmetry carrying the rotor counterparts of the stators. Then the necessary restraints of the degrees of freedom are usually achieved by providing a set of upper radial bearings restraining radial motions of an upper portion of the shaft in two mutually perpendicular radial directions, a set of lower radial bearings restraining radial motions of a lower portion of the shaft in two mutually perpendicular radial directions, and a set of axial bearings (often called thrust bearings) restraining axial motions of the shaft. This provides control over five degrees of freedom.
All or only part of these bearings may be active bearings. In order to control the active bearings, sensors are needed that determine the displacement of the shaft from its ideal position along particular directions. Such sensors are well known in the art.
Normally, the sensors, the bearings, and the drive motor are arranged together to form a bearing device. The arrangement of components of the bearing device is usually in a particular order. Starting on the upper end of the device, i.e., on the end where the pump rotor with blades is mounted, the order is usually from top to bottom as follows:    a. Upper radial sensor    b. Upper radial bearing    c. Drive motor    d. Lower radial bearing    e. Lower radial sensor    f. Axial bearing    g. Axial sensor
The axial sensor is usually mounted below the lower end of the shaft. Furthermore, additional sensors may be present, e.g., for monitoring the rotation frequency. Such an arrangement is disclosed, e.g., in U.S. Pat. No. 6,465,924 to Maejima (see, e.g., FIG. 5 of that document).
However, such an arrangement leads to relatively large lengths of the bearing device and, consequently, of the shaft. Furthermore, sensors are present in at least three planes along the axis, which makes cabling complex and expensive. Due to the large length of the shaft, moments of force are rather large, necessitating strong bearings. Moreover, long shafts tend to have low bending eigenfrequencies, hampering magnetic bearing control. Finally, a TMP length as short as possible is desirable for the end user.
US Patent Application Publication No. 2003/0155829 discloses a magnetic bearing device having a single thrust disk that is reacted against for both radial and axial displacements. While this design leads to a short device length, it offers active control of only three degrees of freedom.